1. Field of the Invention
This invention relates to an alkaline storage cell and a hydrogen storage alloy for the negative electrode of the alkaline storage cell.
2. Description of the Related Art
There is a great demand for the alkaline storage cell using a hydrogen storage alloy for the negative electrode, for consumer use, because of its characteristics such that it-has a high capacity and that it is cleaner compared with cells using lead or cadmium.
For this type of alkaline storage cell, AB5-type (CaCu5-type) hydrogen storage alloy such as LaNi5 is generally used. However, the discharge capacity of the cell using this type of alloy already exceeds 80% of the theoretical capacity, so that the possibility of further enhancing the capacity is limited.
Thus, in order to enhance the capacity, the development of an alkaline storage cell using rare earth-Mg—Ni hydrogen storage alloy obtained by replacing the rare-earth elements in the AB5-type hydrogen storage alloy partly with Mg has been being advanced. While this type of hydrogen storage alloy can store a large amount of hydrogen, it has problems that it does not easily release the hydrogen stored, and that the corrosion resistance to the alkaline electrolyte is low. Due to these problems, the alkaline storage cell using rare earth-Mg—Ni hydrogen storage alloy for the negative electrode has problems that the discharge characteristic is not good and that the cycle life is short.
In this connection, Japanese Unexamined Patent Publication No. 2002-164045 discloses a rare earth-Mg—Ni hydrogen storage alloy of composition expressed by the following general expression and conditional expression:(R1-a-bLaaCeb)1-cMgcNiZ-X-Y-d-eMnXAlYCodMe c=(−0.025/a)+f where R represents at least one element chosen from a group consisting of Ca and the rare-earth elements including Y (except for La and Ce), M represents at least one element chosen from a group consisting of Fe, Ga, Zn, Sn, Cu, Si, B, Ti, Zr, Nb, W, Mo, V, Cr, Ta, Li, P and S, and atom ratios a, b, c, d, e, f, X, Y and Z are defined as 0<a≦0.45, 0≦b≦0.2, 0.1≦c≦0.24, 0≦X≦0.1, 0.02≦Y≦0.2, 0≦d≦0.5, 0≦e≦0.1, 3.2≦Z≦3.8 and 0.2≦f≦0.29.
Regarding this hydrogen storage alloy, it is thought that when the relationship c=(−0.025/a)+f is satisfied in the general expression, hydrogen is easily released, which leads to an improvement of the discharge characteristic of the alkaline storage cell. Further, it is thought that due to this relationship, precipitation of undesired crystal phases except for Ce2 Ni7 structure, CeNi3 structure and structures similar to these is suppressed and a decrease in the amount of hydrogen stored is prevented, which leads to an improvement of the cycle-life characteristic of the alkaline storage cell.
Further, in this hydrogen storage alloy, by setting Y indicating the proportion of Al to be greater than or equal to 0.02 in the general expression, the oxidation of the alloy is suppressed, while in order to suppress the precipitation of undesired crystal phases, Y is set to be less than or equal to 0.2.
However, also the rare earth-Mg—Ni hydrogen storage alloy disclosed in Japanese Unexamined Patent Publication No. 2002-164045 is not sufficient in the corrosion resistance to the alkaline electrolyte. Thus, an improvement of the cycle characteristic of the alkaline storage cell using rare earth-Mg—Ni hydrogen storage alloy is desired.